Well I just put 5 very rare 2x4 sparkling trans-clear new plates onto Ebay... and in doing so, I did some investigating...

Trans-clear plates were first introduced in the 1962-65 518 (2x4), 519, (2x3), 520 (2x20 and 521 (1x1 and 1x2) architectural parts pack in the early-to-mid 60s, and were made of Cellulose Acetate and had a tendency to warp very slightly and also turn to a Champagne color. A few other spare parts packs also had these small plates in the 1960s.

Then in the 1970s thru early 1990s there were at least a dozen or so sets that had these parts in at lest 1 of these 3 sizes of trans-clear plates, (usually only 1 or 2 in the set)... but they were a dull finish, almost cloudy color trans-clear.

Then fast forward about a dozen years... and today trans-clear plates are again available... but in very small numbers, but not from LEGO sets, but likely from Model Shop surplus that went to PAB walls for sale. The 2x2, 2x3 and 2x4 trans clear plates of today are a very sparklingly clear trans color that shine brilliantly (almost diamond-like).

So when buying these 2x2, 2x3 and 2x4 trans-clear plates (none were made larger)... be aware of which decades they came from. Mixing and matching different plates from different eras is not a desirable look. And yes the modern shiny clear ones are the rarest.

Ironically this was also the case with 1x3 and 1x4 trans clear plates, which were first introduced with the Scandinavian Line Ferry promotional sets in the 1980s and 1990s, and are also Model Shop limited inventory parts today. But not the1x1 and 1x2 trans-clear plates. These were produced in vast quantities, and are a dime a dozen, even new ones. Their available populations number into the hundreds of thousands!

So sometimes TLG does some odd things that we sometimes shake our head and ask "why?".... small trans-clear plates is one of those anomalies!